VLF (very low frequency) communication stations are often used by long-range automated navigation systems as a source of high stability, long-wave, worldwide reference signals. Such navigation systems make use of the phase, but not the data content, of the VLF signals. In the past, VLF stations have used the FSK (frequency shift keying) format to encode data. Recently, however, such stations have begun to alter their transmissions to the more efficient MSK (minimum shift keying) format. The use of the MSK format results in data dependent 180.degree. phase reversals of the transmitted signal. Such phase reversals introduce ambiguities in the phase measurements necessary to relate the VLF signals to geographic position. Therefore as VLF stations were converted to the MSK format, they were lost for use as navigation signal sources. A method of resolving the phase ambiguity caused by the data dependent 180.degree. phase reversals was required for continued use of the VLF stations as navigation aids.
It is known that the phase ambiguities inherent in the MSK format can be resolved by doubling the frequency of the received VLF signal. The phase measurement of this doubled frequency signal is then independent of the 180.degree. phase reversals. The initial method of performing the doubling in VLF navigation receivers involved the use of an analog multiplying technique with some band limiting and wide dynamic range. Although this method was effective, it resulted in great loss of signal and in frequency instability due to the sensitive nature of the required components. There has therefore been a need to improve upon the analog multiplication technique to reduce these undesirable side effects.